The present invention relates to the thermal cracking and fractionation of hydrocarbons particularly a reduced petroleum crude oil.
The present approach to visbreaking, which is a mild form of thermal cracking as applied to reduced crude or vacuum residue is to pass the stock to be cracked through a heater essentially in the liquid phase where it is elevated in temperature to about 500.degree. C. From there it is fed to a flash fractionator which operates under a small positive pressure of about 2.0 atmospheres, and here, the liquid and vapour phases separate. The vapour phase is then further separated into lighter distillate fractions.
In some well established process solutions, the separated liquid phase from the flash fractionator is further processed in a second flash fractionator which operates under partial vacuum conditions. Here, heavy gas oil is flashed off and recovered as a product or recycled and subjected to more severe thermal working conditions to produce additional lighter distillate products by passing the heavy gas oil through a second heater then a cracking reactor that is essentially free of internals and commonly referred to as a coking or soaking drum.
The fluid passing through the cracking reactor normally flows upwards, and enters essentially in the liquid phase, but leaves in a mixed liquid-vapour phase, depositing coke in the reactor which accumulates as a solid mass. The coke is subsequently removed by cutting it out in lumps and discharging it through a bottom port.
One of the restrictive features of this process solution of visbreaking is that certain large complex molecules, especially the asphaltenes contained in the heavier fractions, have a greater tendency to cause coke deposition in the heater tubes. This results in reduced thermal efficiency and progressive restrictive fluid flow as well as limiting the practical levels of thermal cracking severity.
Another undesirable feature in the case of more severe thermal cracking is that coke removed from the reactor contains substantial quantities of entrained heavy oils and tars in the interstices of the coke mass.